1. Field of the Invention
The present invention relates to a method and an apparatus for transferring a semiconductor wafer to a cassette from a boat in front of the inlet of a furnace, and also relates to a boat which is sent in, and out from, the furnace while carrying a number of stacked semiconductor wafers.
2. Description of the Related Art
A semiconductor device, such as an IC or LSI, is produced by sequentially subjecting a wafer cut out from an ingot to various processes, such as a surface treatment, thermal oxidation, impurity diffusion, film deposition and etching. During these processes, the semiconductor wafer repeatedly undergoes heat treatment several times.
In general, a multistage furnace, e.g., a four-stage furnace, which is furnaces vertically stacked, is used as a heating furnace for heat treatment for semi-conductor wafers. A multi-shelf unit is disposed in front of the furnace inlet, and various automatic units for treating semiconductor wafers are mounted to this unit.
An exclusive wafer boat is used in sending semiconductor wafers in, and out from, the furnace. One boat generally carries a stack of 200 wafers at a maximum per lot.
An exclusive cassette (wafer-carrier) is used to convey semiconductor wafers to a furnace from the position at which the previous process has been effected. One cassette generally carries 25 wafers at a maximum per lot. Accordingly, it is necessary to transfer wafers from a plurality of cassettes onto one boat.
In dealing with wafers, an operator is inhibited from directly touching the wafers with his hands for decontamination. Therefore, an exclusive wafer transferring apparatus is disposed at the proximity of the furnace inlet as an installation of the furnace in order to automatically transfer wafers to the boat from the cassettes.
The wafer transferring apparatus comprises a first stage (carrier stage) on which a cassette is disposed, a second stage (boat stage) on which a boat is disposed, and a loading device for transferring wafers to the second stage from the first one and loading the wafers onto the boat. The loading device includes a pushing mechanism for pushing up the wafer in the cassette and a chuck mechanism for holding and releasing the wafers.
In transferring wafers from a cassette to a boat by means of the transferring apparatus, a plurality of cassettes each carrying wafers are aligned on the first stage, the wafers in the first cassette are lifted above the cassette by the pushing mechanism, these wafers are held and conveyed over the boat on the second stage by the chuck mechanism, and they are then disposed on the boat by releasing the chuck. Substantially the same operation is repeated for the second to sixth cassettes to sequentially transfer the wafers in these cassettes onto the boat to fill it up.
Then, the boat is lifted, by an elevator device, onto the multi-shelf unit in front of the furnace inlet and is inserted in the furnace by the soft landing device. The operation sequence is executed by the transferring device under the control of a computer based on a preset program.
As shown in FIG. 1, conventional boat 1 includes a pair of end plates 1a and 1b, four parallel support rods 2b to 2d supported by the end plates 1a and 1b, and handles 3a and 3b. Each support rod 2a, 2b, 2c or 2d has a number of grooves 4 formed therein at equal intervals to hold wafers, so that boat 1 has a substantially symmetric outline with respect to its length.
The mutual intervals between grooves 4 for holding wafers are the same as those of the grooves formed in the internal wall of each cassette. The surface for holding wafers of the loading device also has grooves at substantially the same intervals as grooves 4.
According to the conventional wafer transferring method, wafers are sequentially placed on boat 1 from the side of one end plate 1a toward the other end plate 1b. In general, all the wafers in each cassette are transferred onto the boat together.
Boat 1 is made of a refractory material, such as quartz-glass or SiC and generally has a low size accuracy. Since grooves 4 of the boat are sequentially formed, with one end plate 1a as a reference, toward the other end plate 1b, the distance L.sub.1 from one end plate 1a to the adjoining groove 4 differs from the distance L.sub.2 from the other end plate 1b to its adjoining groove 4. That is, strictly speaking, boat 1 is asymmetrical with respect to its length.
Assuming wafers are sequentially transferred onto boat 1 from the side of end plate 1a, therefore, an operator measures the distance L.sub.1 specific to each boat and remembers it prior to wafer transfer. And, for each wafer transfer operation, the operator should input the specific value L.sub.1 to a boat to be used, to a computer and then place the boat on the second stage.
That is, the computer programming for controlling the operation of the transferring device is set with the position of one end plate 1a as the reference.
As many boats are frequently put in and out from the furnace, it is likely that the operator will put boats in a wrong direction or in a wrong order.
In the case where a boat is put on the second stage with the wrong side on front, if wafers are sequentially disposed on the boat from the side of the other end plate 1b according to the mentioned programming, there may occur a mismatch between the positions of the wafers grabbed by the chuck and the positions of grooves 4 of the boat.
When the mismatch occurs, the edge portions of the wafers rub against the peripheries of grooves 4 so as to generate quartz-glass powder, thus contaminating the clean room. If the peripheries of grooves 4 are scraped to thereby reduce the accuracy of the grooves, the boat may not be used any more. That is, the life of the boat is shortened.
When the mismatch between the wafers and grooves 4 is prominent, the wafers will not be fitted in their associated grooves 4, damaging the wafers and reducing the yield of the final products.
Further, in a case where an operator will put boats in a wrong order, with a single wafer transferring apparatus shared by four furnaces (a four-stage furnace), there may be a difference between data input to a computer and the actual distance L.sub.1 of the boat, thus causing a mismatch between the wafers and grooves 4.
Further, it is probable that the boat expected to be inserted into the first-stage furnace is actually put in the second-stage furnace.
With the prior art, it is difficult to visually identify boats and the reference side of each boat for wafer transfer, so that an identification mark is put on each boat. However, since the boat is heated at a high temperature in the furnace and is subjected to etching in order to remove the impurities adhered to the boat, the identification mark is likely to become unidentifiable.
Since a boat is deformed by heat, the reference distance L.sub.1 initially set varies. In this respect, it is necessary to measure the reference distance L.sub.1 and input this measurement to a computer every time a boat is inserted in the furnace.
In addition the intervals between the grooves for receiving wafers also vary due to thermal deformation of the boat, so that if wafers are sequentially transferred on the boat from one end thereof, the deviation between the wafers and grooves is accumulated, thus eventually causing a mismatch therebetween.